Gearboxes for wind turbines may comprise at least one planetary gear unit comprising a planet carrier, planet gears, a sun gear and a ring gear. FIG. 1a and FIG. 1b schematically illustrate a planetary gear unit 10 according to the prior art. The planetary gear unit 10 comprises a housing 1 in which a plurality of planet gears 2 and planet shafts 3 and a planet carrier 4 are located. The planet gears 2 are rotatably supported by planet bearings 2a. The planetary gear unit 10 furthermore comprises a ring gear 5 and a sun gear 6 for mutual interaction with the planet gears 2. The planet carrier 4 comprises a backplate 7 with bores 8 through which the planet shafts 3 extend. The planet carrier 4 is connected to the rotor hub (not shown) of the wind turbine and is rotatably mounted in the housing 1 by means of planet carrier bearing 9 (see FIG. 1b).
Gearbox designs are known in which the planet carrier bearing, next to rotatably supporting the planet carrier, also serves as a main bearing for supporting the rotor hub to which the planet carrier 4 is connected. However, using one main bearing to support both the rotor hub and the planet carrier 4 is less rigid than in the more classical designs whereby the rotor hub and the planet carrier 4 are supported by separate bearings. When using one main bearing for supporting both the rotor hub and the planet carrier 4, misalignments may occur between the planet carrier 4 and the ring gear 5 which increase the risk of causing damage to the planet gears 2.
A known solution to this problem is to fix the planet shafts 3 to the planet carrier 4 in a more or less flexible way, as is known in the prior art. However, in this case and under particular circumstances, e.g. under particular loads, the planet shafts 3 may rotate around their own axis and with respect to the planet carrier 4. Rotation of the planet shafts 3 during operation of the gearbox can cause damages. As a consequence, the life time of the planetary gear unit 10 may be decreased.